Abstract Acute Myeloid Leukemia (AML) is characterized by differentiation blockade and the clonal proliferation of myeloid precursor cells that culminates in the failure of normal hematopoiesis. Regardless of intensive research efforts, few therapeutic advancements have been made in past 40 years, and mortality remains high with the overwhelming majority of patients succumbing to disease within five years of treatment. Overexpression of anti-apoptotic protein, BCL-2, is one hallmark of AML. A new class of small molecule inhibitors, referred to as BH3 mimetics, selectively target specific BCL-2 family proteins. Venetoclax (VEN), a selective BCL-2 inhibitor, was recently approved by the FDA in combination with DNA methyltransferase inhibitors for use in elderly AML patients unfit to receive standard chemotherapy, exhibiting response rates up to 70%. Despite these remarkable results, primary and acquired VEN resistance remains a concern. We and others have shown that VEN resistance occurs mainly through the upregulation of another BCL-2 family member, induced myeloid leukemia cell differentiation protein (MCL-1), and that inhibition of MCL-1 leads to apoptosis in VEN resistant cells. However, upon further analyses of these underpinnings, we observed discordance in response to BH3 mimetics in the different hematopoietic compartments of primagraft AML PDX models. BH3 profiling and AML primagraft PDX bone marrow expansion studies using bone marrow mononuclear cells accurately predict patient specific BH3 dependency, but we have seen contradictory results in the peripheral blood. Although peripheral cells are often of the same clonality and differentiation stage as in the bone marrow, our studies indicate that peripheral AML blasts are more sensitive to MCL-1 inhibition in both cell line and patient derived xenografts, regardless of marrow BH3 dependency. Therefore, we hypothesize that peripheral blast cells in AML are more sensitive to MCL-1 inhibition than those in the bone marrow and propose that the use of mobilization agents will increase sensitivity of AML cells to MCL-1 inhibition, potentially reducing the dose of MCL-1 inhibitors needed to achieve response. We will leverage our laboratory?s invaluable resources and expertise to test this hypothesis with the following aims: 1. Assess the BH3 dependent characteristics of AML blasts harvested from the bone marrow and periphery of patients and PDX models and 2. Determine the utility of mobilization as an accessory therapy to enhance BH3 mimetics. These studies will further elucidate the relationship between BCL-2 family proteins in blast cells and different hematopoietic microenvironments and whether altering the microenvironment can affect this relationship. Further, as it is well known that the marrow niche provides proliferative and survival advantages that result in chemotherapy resistance, plerixafor, a CXCR4 agonist, has successfully been used in combination with other agents in AML as a mobilization agent to release cells from the bone marrow into the periphery and delay disease relapse. Importantly, as BH3 mimetics are not without toxicity, these studies will investigate the utility of plerixafor to enhance response to BH3 mimetic based regimens or reduce the dose needed to achieve response in both VEN nave and VEN resistant AML.